This invention relates to electronic security apparatus for detecting intrusion by unauthorized personnel into close proximity with protected objects, such as file cabinets containing secret documents, safes containing valuable articles, etc. More particularly, this invention relates to electronic security apparatus for producing a field by exciting an antenna formed by connecting the protected objects together and for sensing a disturbance in the field due to an intrusion to activate an alarm.
The prior art discloses various electronic security apparatus with circuitry for exciting a protected-object antenna and for sensing a disturbance in the field due to an intrusion, as by sensing a shift in the frequency of an oscillator which excites the antenna due to a change in the capacitive reactance of the antenna when an intrusion occurs. One type of prior art circuitry which is highly sensitive to movement by an intruder is disclosed in Premack, U.S. Pat. No. 3,222,664.
The Premack patent discloses circuitry which includes two oscillators that are synchronized in frequency. The signal of one of the oscillators excites an antenna, such as formed by a perimeter fence. The signals of the two oscillators are fed to a phase detector which produces a phase-difference signal. An instantaneous change in the frequency of the oscillator signal that energizes the antenna, due to a change in the capacitive reactance of the antenna when an intrusion occurs, is reflected by a shift in phase between the two oscillator signals, so that the phase-difference signal produced by the phase detector changes to activate an alarm.
In order to synchronize the two oscillators, a switch is closed to provide a fixed coupling impedance between the two oscillators. Furthermore, AFC through feedback from the phase detector and a low pass filter to control the transconductance of the pentode in the oscillator which energizes the antenna can also be employed to extend the range over which the electronic security apparatus operates. Nevertheless, the fixed coupling impedance always interconnects the two oscillators even when AFC is employed.
Cowen, U.S. Pat. No. 2,907,017 refers generally to the problem of defeatability of electronic security apparatus by an intruder who determines the frequency of the signal which excites the antenna and connects a frequency generator operating at that frequency to the antenna. No specific reference is made to the type of circuit means disclosed in the Premack patent. Since the Premack patent discloses that the two oscillators are bidirectionally coupled by a fixed coupling impedance, however, the Premack electronic security apparatus is defeatable by connection of a frequency generator to the antenna.